The present invention is directed to in-circuit testing. It has particular, although no exclusive, applicability to the identification of defective devices on a common bus.
In-circuit testing is the testing of electronic devices after they have already been connected together in a circuit. To test a given device in an in-circuit test, it may be necessary to "back-drive" nodes driven by other devices in the circuit. Back-driving is forcing a node to the level opposite that to which a device is attempting to drive it. Back-driving requires employing considerably more current than is ordinarily needed to generate signals in the circuit, and it results in dissipation of considerable power in the devices whose outputs are being overcome by the backdriving. The power levels are such that, if sustained indefinitely, they would damage the device in question. In circuit tests are therefore designed so that the backdriving duration is kept below a dangerous level. Thus, back-driving imposes a constraint on the in-circuit test.